This application is related to commonly assigned U.S. patent application Ser. No. 08/414,833, (RD-23,075) filed Mar. 31, 1995, of AR Duggal et al., filed concurrently herewith, the disclosure of which is hereby incorporated by reference.
This invention relates generally to methods of imaging of acoustic energy, more particularly to methods of imaging of acoustic energy using laser-based ultrasound detection equipment and most particularly to methods of imaging acoustic energy using an array of semiconductor microlaser-based ultrasound detection equipment.
Ultrasound equipment is commonly used in medical imaging and for non-destructive evaluation (NDE) of materials. Ultrasound analysis involve the propagation of energy through solids, liquids, and gases as acoustic waves; typically a pulse of acoustic energy is applied to an object to be imaged and reflected waves of the acoustic pulse are detected and processed for imaging and analysis. The spatial relationship and amplitude of the reflected waves provide information as to the location and nature of structures that reflected the acoustic energy in the object being analyzed.
Piezoelectric transducers are frequently used to generate ultrasound pulses transmitted into the object to be analyzed and to detect reflected waves received at the transducer. Piezoelectric devices require extensive electrical cabling which places practical limits on the number of pixels that can be placed in a transducer array, which in turn limits the resolution of the array.
Specifically, when utilizing piezoelectric transducers for ultrasound equipment used in medical imaging, currently a one-dimensional array of 128 elements or pixels are utilized. The piezoelectric pixels are each independently electrically connected by wires to the signal processing unit which would then process the signals received from the transducers to an imaging. In order to improve the ultrasound image quality, it is desired that a two-dimensional array with, such as, for example, 128.times.128 transducers/pixels is needed. Due to the complexity of the many wires required to connect the piezoelectric transducers to the signal processor, and the complexity thereof, to utilize piezoelectric transducers is simply too complex.
Optical techniques have also been used for generation and detection of acoustic waves in ultrasound imaging. For example, energy from a laser beam focused on the surface of an object to be examined can generate an acoustic pulse in the object. The return pulse of acoustic energy is typically detected optically through the use of interferometry. A review of such techniques, is provided in the book Laser Ultrasonics--Techniques and Applications by C. B. Scruby and L. E. Drain (IOP Publishing Ltd 1990), which is incorporated herein by reference. Noninterferometric techniques of optical detection of ultrasound include the knife-edge and surface-grating techniques and techniques based on reflectivity and light filters. See "Optical Detection of Ultrasound" by J. P. Monchalin, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, UFFC-33, September 1986, which is incorporated herein by reference. These laser-based methods of ultrasound detection are much less sensitive, by several orders of magnitude, than conventional piezoelectric-based methods.
Another laser-based method for detecting sound waves has been suggested in the article "Laser Hydrophone" by Y. A. Bykovskii et al., in Sov, Phys. Acoust. 34, p 204, March 1988. In the Bykovskii et al. optical hydrophone, movement of the hydrophone membrane varies the power and / or the phase of a semiconductor laser in the hydrophone to generate changes in the amplitude of an optical signal. The Bykovskii sonar hydrophone is relatively inefficient and thus has low sensitivity.
In a more recent approach to the use of an optical transducer assembly for ultrasound applications, an optical transducer assembly which includes a transducer housing and a signal laser, such as a microcavity laser or a microchip laser that is optically pumped, to detect ultrasound by monitoring the frequency modulation of the laser output caused by the interaction of the ultrasonic disturbance with the laser cavity, was mounted in the transducer housing. This interaction could involve a change in the laser cavity length and/or a change in the index of refraction of the lasing medium. This approach further detailed a scheme using fiber optic cables to reduce the interconnect complexity when using an array of microcavity lasers. The optical frequency generated by the signal laser is modulable (that is, adapted to or capable of being modulated) in correspondence with acoustic energy incident on the transducer assembly. The signal laser comprises an optical cavity in which a lasing medium is disposed, and first and second reflectors that are disposed at respective opposite end surfaces of the optical cavity along an optical path axis of the cavity. The second reflector can alternatively be replaced with a compliant cavity that acts as a Gires-Tournois interferometer. The signal laser is adapted such that acoustic energy incident on the transducer assembly changes the length of a cavity along the optical path axis, or, alternatively, changes the index of refraction in the optical cavity, and such changes result in a substantially linear variation of the optical frequency of light generated by the laser.
In one embodiment, as disclosed in U.S. Pat. No. 5,353,262, assigned to the assignee of the present application, the disclosure of which is herein incorporated by reference, an optical transducer, such as used in an ultrasound system, includes a signal laser which generates an optical signal the frequency of which varies in correspondence with acoustic energy incident on the transducer. An optical cavity in the signal laser is disposed such that incident acoustic energy causes compression and rarefaction of the optical cavity, and this displacement varies optical frequency generated by the laser. A laser pump coupled to the lasing medium is adapted to apply selected levels of excitation energy appropriate to the detection of acoustic pulses. The signal laser alternatively is adapted such that the refractive index of the optical cavity is varied in correspondence with the incident acoustic energy to modulate the optical frequency of the light generated by the signal laser.
In an alternate embodiment, a piezoelectric device is disposed to receive the incident acoustic energy and generate a corresponding electrical signal that is applied to an electro-optic cell in the optical cavity, or alternatively, to conductors to generate an electric field across the lasing medium.
Despite the efforts described above to provide a simpler, more compact ultrasound detection device, a need still exists in medical imaging and nondestructive evaluation (NDE) of materials for an ultrasound transducer interfaced to support equipment which minimizes interconnection complexity. For example, as stated above, current state-of-the-art ultrasound imaging arrays use a linear array of about one hundred separate ultrasound pixels each of which are connected to a separate coaxial cable. Array sizes are currently limited by the complexity of the interconnect cabling. By using an optical interlace between the transducer head and support equipment, cabling can be significantly reduced in size for the same number of pixels. Alternatively, a much larger number of pixels can be connected by the same size cable. Additionally, it would be advantageous for NDE and underwater acoustic sensing applications to be able to detect as wide a frequency bandwidth of acoustic signals as possible. Finally, ultrasonic microscopic imaging of biological and micro-electronic structures requires an array of small, high bandwidth ultrasonic detectors in contact with the sample being imaged.